The preparation of polyvinyl imidazole-functionalized magnetic biochar decorated by silver nanoparticles as an efficient catalyst for the synthesis of spiro-2-Amino-4H-pyran compounds

The silver nanoparticle was synthesized by developing poly (1-vinylimidazole) on the surface of magnetized biochar (the stem and roots of Spear Thistle) (biochar/Fe3O4/PVIm/Ag). This nanocomposite was characterized by Fourier-transformed infrared spectroscopy (FTIR), powder X-ray diffraction (XRD), vibrating sample magnetometer (VSM), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM–EDS), and transmission electron microscopy (TEM). The SEM and TEM images of the nanocatalyst, biochar/Fe3O4/PVIm/Ag-NPs, confirmed the observation of microscopic sheets of biochar. The catalytic activity of these Ag NPs was tested via multicomponent reaction plus reusing to successful formation of 2-amino-4H-pyran and functionalized spirochromen derivatives. The prepared nanocatalyst was easily separated by an external magnet and reused in repeating coupling reaction cycles four times without remarkable activity loss. The catalyst showed great efficiency and reusability, thus making it an ideal candidate for catalytic purposes in several organic transformations.

www.nature.com/scientificreports/ Heterocyclic compounds 31,32 synthesized through MCR 33 in the presence of heterogeneous catalysts in an aqueous medium are of particular importance 34,35 .
A silver nanoparticle was synthesized by developing poly(1-vinylimidazole) on the surface of magnetic biochar (biochar/Fe3O4/PVIm/Ag). The catalytic activity of the heterogeneous catalyst was investigated for the synthesis of spiro-2-Amino-4H-pyrans (spirochromenes) through multi-component reactions. The reusing test confirmed that the catalyst had relative stability and reusability, making it a good candidate for catalytic purposes. The catalyst can be reused several times in repeating the coupling reaction cycles with some loss of its activity. It should be noted that MCRs have attracted much attention in both the scientific and industrial worlds due to their specific engineering viability, intrinsic particle economy, and achievable integration [42][43][44] (Scheme 1).

Experimental
Materials. All chemical substances have been used as bought from Sigma-Aldrich and Merck Companies.
Biomass material. The raw biomass material in this paper was the stem and roots of Spear Thistle (Fig. 2).
Firstly, the material was washed several times using deionized water to remove the impurities. Then, the stem and roots of Spear Thistle were crushed within a particle size range of 0.9-2 mm. After that, the samples were poured into a Teflon-lined hydrothermal autoclave reactor, then the deionized water ( Tables 1 and 2. This reaction was monitored using TLC (n-hexane/ethyl acetic acid, 3:2). After the completion of the reaction, it was cooled to room temperature, and the prepared nanocatalyst was isolated via an external magnet. The obtained product was dried and crystallized in hot ethanol.
In addition, to ensure the reproducibility of the catalyst in optimal conditions, the reaction was performed three times, and the nanocatalyst was tested. The yields presented are the average of three replicates.   Fig. 4. The morphology of biochar/Fe 3 O 4 /PVIm/Ag was seen in FE-SEM images with small particles of Fe 3 O 4 and Ag with almost spherical morphology, which can be recognized are stuffed together and dispersed on the biochar surface. Additionally, the size of synthesized particles is 33.29 nm to 50.12 nm, which recommends they're nanosized.

Results and discussion
The EDX examination of the nanocatalyst, as appeared in Fig. 5, further illustrates the presence of Ag, Fe, C, O, and N elements as well as confirms the successful immobilization of Ag nanoparticles in biochar/Fe 3 O 4 /PVIm.
Furthermore, elemental mappings of biochar/Fe 3 O 4 /PVIm/Ag nnanocatalysts were provided to characterize the catalyst. As shown in Fig. 6, Ag atoms were uniformly dispersed in the catalyst. It showed that the distribution of C and Fe atoms was uniform, therefore elements were uniformly distributed in biochar.
X-ray diffraction (XRD) pattern was performed to approve the crystalline structure of the synthesized biochar/Fe 3 O 4 /PVIm/Ag nanocatalyst (Fig. 7). The biochar displayed a broad peak at 22° (002) related to graphitelike structures (crystalline carbon). The characteristic peaks of Ag nanoparticles were seen at 2θ = 38.     www.nature.com/scientificreports/ To assist decide catalyst properties, the magnetic property of biochar/Fe 3 O 4 /PVIm/Ag nanocatalyst, containing a magnetite component was considered by a VSM at ambient temperature (Fig. 8). As illustrated in Fig. 8, the maximum saturation magnetization (Ms) value of biochar/Fe 3 O 4 was estimated to be 38.7 emu g −1 (Fig. 8a) indicating that it has superparamagnetic properties. The Ms was obtained at 38.3 (Fig. 8b) and 33.0 (Fig. 8c) amu g −1 for biochar/Fe 3 O 4 /PVIm and biochar/Fe 3 O 4 /PVIm/Ag, respectively, which shows that the saturation magnetization has decreased with the addition of non-magnetic materials. But this reduction is slight, so the nanocatalyst is easily separated by a magnet from the reaction media.
The gotten magnetic hysteresis circle pictorially appears in Fig. 8. It is readily apparent that it is without a hysteresis circle (S-shaped). It is worth noting that the biochar/Fe 3 O 4 /PVIm/Ag can be effortlessly collected by utilizing a magnet, and as a result, the catalyst recovery is facilitated, improving its recyclability.
The TEM image (Fig. 9)  Inductively coupled plasma was used for the examination of Ag content as the main nanocatalyst component. According to the results, the silver content was 0.051%. Comparing the results of ICP and EDS shows the difference between the amount of silver in the catalyst and this is because EDS is a surface and local analysis and gives the amount of silver on the surface, but ICP is a bulk analysis and determines its amount in the whole catalyst.
After the characterization the of synthesized nanocatalyst, which proceeded in eco-friendly chemical processes, we considered the biochar/Fe 3 O 4 /PVIm/Ag as a suitable, easily separable nanocatalyst for a one-pot process of spiro-2-amino-4H-pyran (spirochromene) by the multicomponent reaction (Scheme 1).
To optimize different factors affecting the reaction, different parameters which include catalyst amount, solvent type, and temperature were investigated as a selected reaction including isatin, malononitrile, and dimedone ( Table 1, and Scheme 2).
The required factors were given in high yield, regardless of the influence of the properties of the substituents of isatin. The less spiro-4H-pyran (12a-j) was produced when acetyl naphthoquinone (13) was used (Table 3). It    www.nature.com/scientificreports/ is recognizable that the reaction with ethyl cyanoacetate enforced an extended response time than the reaction via malononitrile, which is probably due to its low reactivity ( Table 3).
The method illustrates to a normal successive reaction wherein the isatin (9), first, connected to malononitrile (10) combines with isatylidene malononitrile in the presence of biochar/Fe 3 O 4 /PVIm/Ag in EtOH/water. This step was named fast Knoevenagel condensation.
Intermediate (17) was formed via a Michael addition mechanism by adding compound (11) to compound (16). Then the hydroxyl group attacked the cyanide group through an intermolecular reaction in compound 18 and finally after tautomerization the final product (19)  To study the efficiency of the synthesized nanocatalyst, its catalytic performance was compared with the others mentioned in Table 4  Moreover, our strategy achieved the required items in a way well yielded and fast response times. In terms of green chemistry, the reusability of this nanocatalyst, in conjunction with utilizing H 2 O/EtOH as an almost green solvent, enables this environmentally friendly and harmless catalyst to be used in mechanical systems.
The relevant synthesis in this research has already been reported in the literature. All steps have been carried out according to organic synthesis principles, such as Michael addition, water removal, and tautomerization 71,72 . Reusability of catalyst. We also investigated the importance of the catalyst and its potential for reuse. The reaction between isatin, barbituric acid and malononitrile under optimized conditions was selected to investigate the reusability of the synthesized nanocatalyst. After the end of the reaction, the catalyst was isolated thru an external magnet and washed with ethanol. The separated nanocatalyst was reused inside the next cycle in the similar reaction environment. In this study was observed that the nanocatalyst can be recovered and reutilized in at least 4. In the fourth cycle, the efficiency of the catalyst decreased (Fig. 10, and Scheme 7).
To study the stability of the nanocatalyst in the optimal reaction conditions, ICP was taken and the amount of silver decreased from the initial value of 0.051-0.042%. This result shows that the catalyst has relatively acceptable stability.

Conclusion
In conclusion, the above-presented investigation made accessible an efficient and quick approach for the synthesis of Ag nanoparticles immobilized onto the magnetic biochar/polyvinyl imidazole to get an eco-friendly nanocomposite with a good activity, green, and heterogeneous catalyst. The possibility of obtaining and compositing three materials, biochar, polymer, and nanoparticles, has the ability to access a green compound in the imminent need for the design of efficient and environmentally friendly catalysts. Biochar/Fe 3 O 4 /PVIm/Ag as a new catalyst was effectively utilized as a nanocatalyst within the production of spirochromenes. This catalyst was used to obtain the desired products with high yield. The biochar/Fe 3 O 4 /PVIm/Ag nanocatalyst was separated easily by an external magnet. The catalyst was recoverable and reusable for 4 runs without a significant reduction in effectiveness. Another advantage of this catalytic system had been that it could be carried out under gentle reaction conditions in very brief times, in conjunction with a simple process, and the good stability under the optimized conditions. In summary, as the technical advantages of nanotechnology rapidly change from laboratory to large-scale industrial development, nanomaterials are used in all synthesis applications.